Semiconductors that contain nitrogen (N) as the Group V element are excellent candidates as useful materials for short-wavelength light emitting devices because of their wide band gap. Among these, extensive research has been conducted on gallium nitride based compound semiconductors (GaN based semiconductors: AlGaInN), and blue and green light emitting diodes (LED) have already been put to practical use.
FIG. 8 is a cross-sectional view showing a currently in use GaN based ultraviolet light emitting diode. The light emitting diode is fabricated in the manner as described below wherein “u-” stands for undoped, “p-” stands for p-type, and “n-” stands for n-type:
First, a u-GaN seed crystal 42 is grown on a sapphire substrate 41 using metalorganic vapor phase epitaxy (MOVPE) techniques. Next, SiO2 is deposited by chemical vapor deposition (CVD) or the like on the seed crystal 42. Subsequently, the SiO2 layer is processed by photolithography and etching to form dielectric masks 54 with a stripped pattern. Then, to obtain a flat surface, u-GaN 43 is regrown by an epitaxial lateral overgrowth (ELO) technique from starting points at portions of the seed crystal 42 that are exposed through dielectric masks 54. In this case, as the growing method, MOVPE techniques, hydride vapor phase epitaxy (HVPE) techniques, and the like may be employed. Subsequently, an n-GaN contacting layer 44, u-GaN 45, InGaN active layer 46, p-AlGaN gap layer 47, and p-GaN contacting layer 48 are sequentially deposited.
A mask having a predetermined shape is then formed on the surface of the p-GaN contacting layer 48, and then etched to expose a portion of the n-GaN contacting layer 44. Then, a transparent p-type electrode 49 is formed on the p-GaN contacting layer 48, and on the exposed portion of the n-GaN contacting layer 44, an n-type electrode 53 is formed. Finally, on the p-type electrode 49, a base electrode 50 is positioned and a gold wire 52 is bonded to the base electrode 50 via a solder ball 51. In the same manner, a solder ball 51 and gold wire 52 are also attached to the n-type electrode 53.
In this light emitting diode, the p-type electrode 49 is formed from a transparent and conductive thin film, wherein the p-type electrode 49 thereof becomes an emission detection surface by applying current from the base electrode 50 to the entire surface of the p-type electrode 49. By making the p-type electrode 49 side to be the emission detection surface, wire bonding becomes possible. As a result, compared to a face-down mounting method, wherein bonding is conducted on the p-type electrode side by turning the device upside-down, this method is advantageous in that the device can be miniaturized and there is no need for accurate alignment, improving productivity.
As a substrate for GaN based crystal, sapphire, SiC, NGO, etc., are used; however, none of these substrates have the lattice constant that matches that of GaN, making it difficult to obtain coherent growth. Therefore, in a GaN layer that has been grown on such a substrate, a large number of dislocations (edge dislocations, screw dislocations, mixed dislocations) exist. For example, when a sapphire substrate is used, there exist approximately 1×109 cm−2 dislocations. These dislocations decrease the luminous efficiency of an ultraviolet light emitting diode.
As a method for decreasing the dislocation density, the above-described light emitting diode employs the epitaxial lateral overgrowth technique to deposit a GaN layer. This method is effective in decreasing the number of threading dislocations in a system having a large lattice mismatch.
FIG. 9 is a partial enlarged view of FIG. 8 schematically showing the distribution of the dislocations in a GaN crystal that has been obtained by ELO. As shown in this figure, in the u-GaN layer 43, a large number of dislocations exist in the region X1 located above the seed crystal 42, wherein the dislocation density thereof is approximately 1×109 cm−2. In contrast, the region X2 located on the dielectric mask 54 has fewer dislocations, wherein the dislocation density thereof is decreased to approximately 1×107 cm−2. In this GaN crystal, the width of the dielectric mask 54 is approximately 4 μm and the interval therebetween is approximately 12 μm. As described above, employing ELO makes it possible to form a crystal on the dielectric mask 54 that has a low dislocation density, reducing the crystal defects and improving the luminous efficiency of the ultraviolet light emitting diode. An example of a semiconductor light emitting device that has a region of low dislocation density, other than that described above, is disclosed in the specification of the European Patent Publication No. 1104031.
However, a light emitting diode having such a structure, in which the emission detection surface is on the p-type electrode side, renders a problem in that light from the active layer cannot be effectively emitted because the emission light to the n-type electrode, located opposite the p-type electrode, scatters or is absorbed.
In order to solve this problem, a light emitting device that incorporates a reflecting mirror called a Bragg reflecting mirror therein is proposed. In this light emitting device, light that travels from the active layer toward the n-type electrode is reflected outside the device by the reflecting mirror. However, even in this structure, the luminous efficiency is not satisfactory. Furthermore, because it requires a step to incorporate a reflecting mirror, this structure has a drawback in that the fabrication process becomes complicated.
The present invention aims at solving the above drawbacks and providing a semiconductor light emitting device that comprises a p-type electrode serving as an emission detection surface and that achieves high luminous efficiency; and providing a method for fabricating the same.